<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <div class="'B">
    <div cid='demo'>你好</div>
    <div cid='demo'>我好</div>
  </div>
  <script>
    let A = document.querySelector('.B')
    console.log(A.getAttribute('cid'))
  </script>
</body>

</html>